Gas detection sensor

ABSTRACT

A gas detection sensor permits precise measurement of the concentration of flammable gas in a detection or subject gas and the concentration of oxygen in a detection gas containing flammable gases. In the sensor, the heating of the sensor by contact catalytic reaction of flammable gas gives off a detection signal of the flammable gas. The gas detection sensor has a first detection sensor including a diaphragm having a platinum coat on a side which the flowing detection gas comes in contact with, and a thermocouple having respective ends of two different metals placed close to each other and fixed on the side of the diaphragm not coming in contact with the flowing detection gas and which is heated by the contact catalytic reaction of flammable gas. A second detection sensor similar to the first detection sensor detects the temperature of the flowing detection gas.

FIELD OF THE INVENTION

The present invention relates to improvements in a detection sensor forflammable gases and oxygen gas in flammable gases. More particularly,the present invention relates to the sensor for such uses as in securingand ensuring the safety of a variety of production equipment andfacilities and detection of hydrogen gas in pure water for manufacturingsemiconductors and hydrogen gas in gases for manufacturingsemiconductors.

BACKGROUND OF THE INVENTION

Among the detection sensors for flammable gases that have been usedwidely are the contact catalytic reaction type (or the contactcombustion type) gas detection sensor, semiconductor type gas detectionsensor and thermal conductivity type gas detection sensor. Of thosetypes, the contact catalytic reaction type gas detection sensor hasfound wide uses for detection of such gases as hydrogen gas because ofits long service life and reliability.

FIG. 10 shows a partially broken way view of an example of the sensorelement A of the prior art contact catalytic reaction type gas detectionsensor. This sensor element A includes a coil B of platinum wire some 20μm in diameter with which a mixture of a binder and alumina or silicaalumina to be a catalyst support C is sintered, with a catalyst D likeplatinum supported therein.

The aforesaid sensor element A is incorporated in a bridge circuit fordetection of gas concentration. That is, a bridge circuit is formed, asshown in FIG. 11, with the sensor element A and a temperaturecompensation element Ao made by sintering an inert substance. A specificvoltage is applied to the sensor element A to raise before hand thetemperature to not lower than some 250° C. If a flammable gas likehydrogen gas comes in contact with the preheated sensor element A, thegas will undergo a contact catalytic reaction by the catalytic action ofcatalyst D and the sensor element A will be heated. That increases theelectrical resistance of the sensor element A to break the equilibriumin the bridge circuit and to cause an electric potential difference. Asa result, an indicator E turns. The extent of the turn of the indicatorshows the heating value of the sensor element A, that is, theconcentration of the flammable gas within the detection gas or subjectgas under test.

The sensor element A as shown in FIG. 10 has such advantages as (a) highselectivity for flammable gases, (b) hardly influenced by theco-existing H₂O and (c) suitable for measurement of gas concentrationclose to the lower limit of explosion (in the case of hydrogen gas, 1 to4%).

However, the problem with the sensor element A as shown in FIG. 10 isthat the temperature of the sensor element A has to be maintained at notlower than 250° C. and that the working temperature further rises indetection of the flammable gas concentration. That could ignite theflammable gas. To ensure the safety, the sensor element A has to be madeexplosion-free by covering the sensor element A with such as a wirenetting with a mesh of some 200 or sintered metal. In other words, thesensor element A as shown in FIG. 10 has a serious safety problem.

It is also noted that this type of sensor element A is so formed thatcatalyst D is supported within the catalyst support C as mentioned, andhas a basic problem about the stability of catalytic activity.Especially, the effects on the catalyst in the sinter material by theburning of the flammable gas and the effects on the catalytic activityof carbon coming from incomplete combustion of the flammable gas havennot been elucidated well yet. Few studies are reported in which thistype of the sensor element A is used for detection of a flammable gaswhich is present in small quantities in highly concentrated H₂O or O₂.

Another problem is that it is difficult to clean the inside of thecatalyst support C of this kind of the sensor element A. For thisreason, the sensor element A can not be used in the semiconductormanufacturing process where a high degree of cleanliness is required.

As set forth above, the application of the contact catalytic reactiontype flammable gas detection sensor A for detection of a flammable gaspresent in small amounts in highly concentrated H₂O or O₂ presentsproblems with regard to reliability and the like. It is also the casewith the use of the semiconductor type flammable gas detection sensorand the thermal conductivity type flammable gas detection sensor.Furthermore, while it is possible to use this kind of the sensor elementA as sensor for detection of oxygen in the flammable gases in principle,it has not been put to practical use because of the aforesaid problemslike reliability, and few application studies have been reported.

In other words, the prior art flammable gas detection sensor A of thecontact catalytic reaction type drops substantially with lapse of timein catalytic activity, that is, H₂ gas detection sensitivity. Forreasons of poor reliability, the sensor element A can hardly be appliedfor such uses as detection of the concentration of unreacted hydrogengas in the moisture take-out line of the moisture generating reactor forsemiconductor manufacturing facilities. The same is the case with theaforesaid semiconductor type sensor for detection of hydrogen gas andthe thermal conductivity type sensor for detection of hydrogen gas,which has been confirmed in experiments.

In the moisture generating reactor for semiconductor manufacturingfacilities, it can happen that moisture is generated with excessivesupply of hydrogen gas. In such a case, it is necessary to detect theconcentration of unreacted oxygen gas in the generated moisturecontaining hydrogen in the moisture take-out line. The prior art sensorelement A can not be used in such cases.

Meanwhile, the applicants of the present application developed aflammable gas detector as shown in FIG. 12 that solved the problems withthe prior art contact catalytic reaction type sensor element A fordetection of flammable gases and disclosed the same in unexaminedJapanese patent application No. 9-186383.

This flammable gas detector is composed of a flammable gas detectionsensor 20 and a detector unit 30. The flammable gas detection sensor 20is formed of a first detection sensor 21 provided with a platinumcoating catalyst, a second detection sensor 22 to detect the temperatureof the detection gas (the gas to be detected) or subject gas under testand a sensor holder 23.

The detector unit 30 includes a first temperature detector 31 to detectthe temperature signal from the first detection sensor 21, a secondtemperature detector 32 to detect the temperature signal from the seconddetection sensor 22, a first temperature display 33 and secondtemperature display 34 to display the temperatures detected by theaforesaid two temperature detectors respectively, a temperaturedifference detector 35 to detect the difference between the detectedtemperatures and a temperature difference display 36 to display thetemperature difference from the temperature difference detector 35.

The flammable gas detection sensor 20 is placed in a T-shaped branchpipe 39 with the sensor holder 23 fitted in air-tight and with the twosensor elements 21, 22 held in a gas feeder pipe 37 as shown in FIG. 13.The T-shaped branch pipe 39 is provided with explosion proof metalmeshes 38 in gas feeder pipe 37.

The flammable gas detector shown in FIG. 12 and FIG. 13 is excellent inresponsiveness and gas concentration detection accuracy, and can correctthe detected value without difficulty when the flow rate of thedetection gas changes. Another practical advantage is that the changewith lapse of years in detection sensitivity is relatively small.

But this flammable gas detector has a number of problems yet to besolved. Among the problems requiring urgent solution are contaminationof the high-purity gas flowing through the pipe, the reliability ofdetection precision and safety.

To be specific, the first detection sensor 21 and the second detectionsensor 22 to be placed in the high-purity gas are thermocouples, and theouter surface of the first detection sensor 21 to be heated by thecontact catalytic reaction of flammable gas is coated with a platinumcatalyst film via a barrier coat like TiN.

However, the adhesion strength between the metal, for example,chromel-alumel, forming the thermocouple and the barrier coat like TiNundergoes a change relatively fast with lapse of years. As a result, theplatinum coat on the first detection sensor 21 could fall off,contaminating the high-purity gas. In other cases, partial peeling offof the platinum coat could reduce the catalytic reactivity.

The detection sensors may be formed of platinum. An example is a gasdetection sensor that has the thermocouple of the first detection sensor21 made of noble metals like platinum and rhodium and that has thesecond detection sensor 22—to detect the temperature of the fluid—alsomade of platinum and rhodium with the outer surface coated with abarrier coat like TiN. In this case, there is no fear that the barriercoat will come off with lapse of years.

However, the thermocouple of noble metals would be relatively expensiveand present problems with mechanical strength and machining, which wouldmake it difficult to put it to practical use.

The present invention address the following problems with the gasdetection sensor of the type shown in FIG. 12: (a) the thermocouple madeof base metals is liable to deteriorate in adhesion between the platinumcoat and the thermocouple forming material with passage of years, withpeeled off platinum coat contaminating the high-purity gas and reducingcatalytic reactivity, (b) the thermocouple made of noble metals isexpensive to manufacture, and (c), in the latter case, difficulty inmachining and relatively low mechanical strength, making it difficult toreduce manufacturing costs.

It is accordingly an object of this invention to provide a gas detectionsensor that is free from contaminating the high-purity gas and fromchanging with passage of time in detection precision, yet is excellentin safety and relatively inexpensive to manufacture.

SUMMARY OF THE INVENTION

The inventors have been engaged in developing reactors for generatingmoisture for many years, and in the course of the research anddevelopment they have succeeded in stabilizing the platinum coat formedon the inside wall of the reactor made of stainless steel, that is,preventing the catalytic performance from changing with time.

Then, the inventors have noticed that it is possible to build aflammable gas sensor with little deterioration in catalytic performanceand with high reliability and safety at low costs if the technique forforming a platinum catalytic layer in the reactor for generatingmoisture is applied to a flammable gas detector.

On the basis of that finding, the inventors, thinking that the change inoutput caused by a change in the temperature of the thermocouple shouldbe a factor in detecting the flammable gas concentrations, testedvarious characteristics of thermocouples of different types and studiedthe results in detail.

The present invention was made through such a process. The inventioncomprises a gas detection sensor wherein flammable gas detection signalsare issued by the heating of the sensor caused by the contact catalyticreaction with the flammable gas. The gas detection sensor includes afirst detection sensor having a diaphragm with a platinum coat on thegas contact surface coming in contact with the flowing detection gas,and a thermocouple having the one ends of two metal pieces of differentkinds placed close to each other and fixed on the side of the diaphragmnot coming in contact with gas—the reverse of the side that comes incontact with the detection gas and which is heated by the contactcatalytic reaction of flammable gas—; and a second detection sensorwhich includes a diaphragm coming in contact with the flowing detectiongas and a thermocouple having the one ends of two metal pieces ofdifferent kinds placed close to each other and fixed on the side of thediaphragm not coming in contact with gas—the reverse of the side thatcomes in contact with the detection gas—and which detects thetemperature of the flowing detection gas.

An object of the invention is to provide a gas detection sensor whereindetection signals for oxygen gas in the flammable detection gas areissued by the heating of the sensor owing to contact catalytic reactionwith the flammable gas. This gas detection sensor comprises: a firstdetection sensor which includes a diaphragm having a platinum coat onthe gas contact surface coming in contact with the flowing detection gasand a thermocouple having the one ends of two metal pieces of differentkinds placed close to each other and fixed on the side of the diaphragmnot coming in contact with gas—the reverse of the side that comes incontact with the detection gas—and which is heated by the contactcatalytic reaction of flammable gas; and a second detection sensor whichincludes a diaphragm coming in contact with the flowing detection gasand a thermocouple having the one ends of two metal pieces of differentkinds placed close to each other and fixed on the side of the diaphragmnot coming in contact with gas—the reverse of the side that comes incontact with the detection gas—and which detects the temperature of theflowing detection gas.

A further object of the invention is to provide a sensor as describedabove wherein the diaphragms in the first detection sensor and thesecond detection sensor are made of stainless steel and wherein abarrier coat is formed on the sides of the two diaphragms that come incontact with the gas.

According to the invention, the barrier coat is formed of an oxide ornitride and/or each thermocouple may be made of chromel-alumel.

A further object of the invention is to provide a sensor as describedabove wherein the diaphragms of the first detection sensor and thesecond detection sensor are fit into the detection sensor insertionports of the stainless steel sensor block, with the gas-contact surfaceof the diaphragm facing the gas passage and with the inserting portssealed airtight with the respective diaphragms. The stainless steelsensor block is each provided with an inlet and an outlet for detectiongas, a gas passage through which the inlet communicates with the outletand the first detection sensor inserting port and the second detectionsensor inserting port which communicate with the gas passage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a flammable gas detection sensor of thepresent invention.

FIG. 2 is a sectional view of the first detection sensor.

FIG. 3 is a sectional view of the second detection sensor.

FIG. 4 is a sectional view of the first detection sensor fit into thesensor block.

FIG. 5 is a block diagram showing another embodiment of the detectorbody of the flammable gas detector.

FIG. 6 is an example plot of sensor output temperature versus H₂concentration of detection sensor.

FIG. 7 is an example plot of sensor output temperature versus H₂concentration of detection sensor.

FIG. 8 is an example plot of sensor output temperature versus O₂concentration of detection sensor.

FIG. 9 is an example response characteristic of the detection sensor ofthe present invention.

FIG. 10 is a partially broken away view of an example of the sensorelement of the prior art contact catalytic reaction type sensor.

FIG. 11 is a circuit diagram of a flammable gas detector in which thesensor element in FIG. 10 is applied.

FIG. 12 illustrates a flammable gas detector as previously disclosed bythe inventors.

FIG. 13 is a schematic sectional view showing the mounting of theflammable gas detector illustrated in FIG. 12.

List of Reference Numerals

A sensor element

B coil

C catalyst support

D catalyst

Ao temperature compensation element

E indicator

A₁ for thermocouple

A₂ metal for thermocouple

G gas sample

1 flammable gas detector

2 detection sensor

3 detector unit

3 a first temperature detector

3 b second temperature detector

3 c first temperature display

3 d second temperature display

3 e temperature difference detector

3 f temperature difference display

4 connection cable

4 a connector

4 b connector

5 first detection sensor

5 a diaphragm base

5 b diaphragm

5 c barrier coat

5 d platinum coat

5 e thermocouple

5 e′ sheath

5 e″ annulus

6 second detection sensor

6 a stainless steel

6 b diaphragm

6 c barrier coat

6 e thermocouple

6 e′ sheath

6 e″ annulus

7 sensor block

7 a gas inlet

7 b gas outlet

7 c gas passage

7 d first detection sensor inserting port

7 e second detection sensor inserting port

7 f metal fittings for pipe connection

7 g metal fitting for mounting the sensor

7 h bolt for mounting the sensor

8 thermocouple holder

8 a barrier coat

8 b platinum coat

9 seal ring

10 potentiometer

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the embodiments of the present invention will be described withreference to the drawings.

FIG. 1 is a block diagram of a flammable gas detector in which aflammable gas detection sensor 2 according to the present invention isapplied. FIG. 2 is a sectional view of the first detection sensor 5.FIG. 3 is a sectional view of the second detection sensor 6. And FIG. 4is an enlarged partially sectional view of the first detection sensor 5fit into the sensor block 7.

Referring to FIG. 1, the flammable gas detector 1 according to thepresent invention comprises the flammable gas detection sensor 2, thedetector unit 3 and the connection cables 4 that connect the detectionsensor 2 and the detector unit 3.

The flammable gas detection sensor 2 includes the first detection sensor5 provided with a platinum coat, the second detection sensor 6 fordetecting the temperature of the gas to be detected and the sensor block7.

The sensor block 7 in the shape of a block is made of stainless steelunder JIS designation SUS316L and has the flammable gas inlet 7 a, thegas outlet 7 b, the gas passage 7 c, the first detection sensorinserting port 7 d, the second detection sensor inserting port 7 e andothers. In FIG. 1, the numeral 7 f indicates metal fittings for pipeconnection, the numeral 7 g metal fittings for mounting the sensors, and7 h bolts for mounting the sensors.

The detector unit 3 includes the first temperature detector 3 a todetect the temperature signal from the first detection sensor 5, secondtemperature detector 3 b to detect the temperature signal from thesecond detection sensor 6, the first temperature display 3 c and thesecond temperature display 3 d that display the detected temperaturesfrom the two detectors respectively, the temperature difference detector3 e to detect the difference between the two detected temperatures, andthe temperature difference display 3 f to display the temperaturedifference from the temperature difference detector 3 e.

The connection cables 4 have each detachable connectors 4 a, 4 b at theends respectively and electrically connect the flammable gas detectionsensor 2 and the detector unit 3.

The embodiment shown in FIG. 1 has the first temperature display 3 c andthe second temperature display 3 d provided in the detector unit 3 whichdisplay the detected temperatures of the two sensors 5, 6 respectively.But that is not restrictive. The detector unit 3 may be provided withonly the display 3 f to show the temperature difference (that is, theconcentration of flammable gas). Also, the temperature differencedisplay 3 f may be provided with a flammable gas concentration alarm(not shown). The detector unit 3 may be arranged in any form.

The first detection sensor 5 comprises, as shown in FIG. 2, thediaphragm base 5 a made of stainless steel under JIS designationSUS316L, the diaphragm 5 b molded integrally with the diaphragm base 5a, the barrier coat 5 c formed on the gas-contact surface (outsidesurface) of the diaphragm 5 b, the platinum coat 5 d formed on theoutside surface of the barrier coat 5 c and the thermocouple 5 e made oftwo different metals whose respective one ends are put close to eachother and fixed on the surface (reverse side) of the diaphragm 5 b notcoming in contact with the gas.

In FIG. 2, the numeral 8 is a thermocouple holder that holds and fixesthe sheath 5 e′ of the thermocouple 5 e via the annulus 5 e″.

The diaphragm base 5 a is made of stainless steel under JIS designationSUS316L and is ring-formed in shape with a shoulder 5 a′ formed aroundthe outer circumferential surface for a seal ring (not shown).

The diaphragm 5 b is formed integrally with the diaphragm base 5 a. Thediaphragm 5 b is originally some 1 mm thick and then polished as bylapping to a thickness of some 0.1 to 0.3 mm with an inside diameter ofsome 10 to 20 mm.

The barrier coat 5 c is formed of TiN some 2 μm thick on the gas-contactsurface of the diaphragm 5 b.

In forming the barrier coat 5 c, the outside surface (gas-contactsurface) of the diaphragm 5 b is subjected to a suitable surfacetreatment to remove a metal oxide layer or passive state film naturallyformed on the surface of stainless steel. Then, a TiN barrier coat 5 cis formed. In the present embodiment, a TiN film some 2 μm thick isformed by the ion plating technique.

The materials suitable for the barrier coat 5 c includes nitride filmlike TiC, TiCN and TiAlN as well as TiN and oxide film like Cr₂O₃ andSiO₂. The thickness of the barrier coat 5 c is preferably 0.1 to 5 μm. Athickness of not larger than 0.1 μm is not effective as barrier, whileif the thickness exceeds 5 μm, it will take long to form a barrier coatand, in addition, the difference in expansion coefficient could causethe barrier coat to peel off or other problems when the film is heated.

Other methods of forming a barrier coat 5 c than the ion platingtechnique may used. They include the physical vapor deposition (PVD)such as ion sputtering and vacuum deposition, chemical vapor deposition(CVD), also hot press method and flame spray coating.

The platinum coat 5 d provided on the gas-contact surface of thediaphragm 5 b of the first detection sensor 5 is a platinum coat some0.2 μm thick formed on the barrier coat 5 c. After the barrier coat 5 cis formed, the platinum coat 8 b is formed thereon. In the presentembodiment, a platinum coat 5 d some 0.2 μm thick is formed by the ionplating technique. The thickness of the platinum coat 5 d is preferably0.1 μm to 3 μm. A coating not thicker than 0.1 μm could not remaincatalytically active for a long time. On the other hand, a coatingthicker than 3 μm would increase the cost of forming the platinum coat 5d and would make little difference in catalytic activity and servicelife. Besides, a thicker coat could peel off when heated because ofdifference in expansion coefficient.

A platinum coating 5 d may be formed not only by the ion platingtechnique but also ion sputtering, vacuum deposition, chemical vapordeposition and hot press. Platinum plating is also possible in case thebarrier coat 13 a is made of an electric conductive material such asTiN.

The thermocouple 5 e is formed of two different kinds of metals A1, A2.The one ends of two metals A₁, A₂ are put close to each other—some 0.1to 0.2 mm apart from each other—and fixed on the reverse side (not thegas-contact side) of the diaphragm 5 b. The respective other ends of thetwo metals A₁, A₂ are drawn out through protection of the sheath 5 e′.

In the present embodiment, it is noted that the thermocouple 5 e is abase metal type alumel-chromel thermocouple 5 e.

The thermocouple holder 8 is to hold and fix the thermocouple 5 e. It isso designed that the sheath 5 e′ of the thermocouple 5 e is held andclamped by fastening to the thermocouple holder 8 the annulus 5 e′provided on the sheath 5 e′ of the thermocouple 5 e.

Needless to say, the thermocouple holder 8 may be of any construction,as long as the holder 8 can hold and clamp the sheath 5 e′ of thethermocouple 5 e.

The second detection sensor 6 is exactly identical with the firstdetection sensor 5 except that no platinum coat 5 d is coated as shownin FIG. 3. In FIG. 3, the numeral 6 a indicates a diaphragm base made ofstainless steel (JIS designation SUS316L), the numeral 6 b a diaphragm,the numeral 6 c a barrier coat, the numeral 6 e a thermocouple, thenumeral 6 e′ a sheath, and the numeral 6 e″ an annulus. The diaphragm 6b, barrier coat 6 c, thermocouple 6 e and others are exactly the same asthose in the first detection sensor 5.

The diaphragm base 5 a and diaphragm 5 b and also the diaphragm base 6 aand diaphragm 6 b shown in FIG. 2 and FIG. 3 are integrally formedrespectively. Instead, the diaphragm base 5 a and diaphragm 5 b and alsothe diaphragm base 6 a and diaphragm 6 b are each formed separately andput together as by welding.

In this embodiment shown in FIG. 2 and FIG. 3, it is so arranged thatthe diaphragm bases 5 a, 6 a and thermocouple holder 8 are provided.Needless to say, the diaphragm bases 5 a, 6 a and thermocouple holder 8may be dropped as long as the diaphragms 5 b, 6 b can be inserted andfixed airtight in the sensor inserting ports 7 d, 7 e in the sensorblock 7 and the thermocouples 5 e, 6 e can be firmly held and clamped.

Furthermore, the thermocouples 5 e, 6 e shown in FIG. 2 and FIG. 3 arebase metal type chromel A₁-alumel A₂ (CA) thermocouples. Needless tosay, thermocouples of other types may be used, includingcopper-constantan (CC) type, and iron-constantan (CI) type.

In the present embodiment, the sensor block 7 is formed in the shape ofa block as shown in FIG. 1 and the first detection sensor 5 and thesecond detection sensor 6 are positioned in such a way as to form aright angle with each other wherein the detection gas G may come incontact with the second detection sensor 6 first and then with the firstdetection sensor 5. Instead, it may be so arranged that the detectiongas G may first come in contact with the first detection sensor 5 andthen with the second detection sensor 6.

In the present embodiment as shown in FIG. 1, furthermore, the firstdetection sensors 5, 6 are so positioned that the flow of the detectiongas G crushes against the two sensors 5, 6. Needless to say, the twosensors 5, 6 may be lined up so that the detection gas G may flow alongthe gas-contact surfaces of the respective diaphragms 5 b, 6 b.

FIG. 4 is an enlarged partially sectional view of the first detectionsensor 5 fit into the sensor block 7. In FIG. 4, the numeral 9 is a sealring, the numeral 7 g a metal fitting for fixing the sensor and thenumeral 7 h a bolt for fixing the sensor. The first detection sensor 5is fit into the first detection sensor inserting port 7 d of the sensorblock 7 with the seal ring 9 placed around the sensor 5 inside the port7 d. Pressed from above by the metal fitting 7 g for fixing the sensor,the first detection sensor 5 moves in until the gas-contact surface—theoutside surface with the platinum coat 5 d formed thereon—is exposed tothe gas passage 7 c. The first detection sensor 5 is fixed to the sensorblock 7 with the first detection sensor inserting port 7 d beingmaintained in an airtight state.

(Operation of the flammable gas detector)

The operation of the flammable gas detector 1 according to the presentinvention will be explained.

Referring to FIG. 1, the detection gas G that flows into the sensorblock 7 from the gas inlet 7 a first comes in contact with the diaphragm6 b of the second detection sensor 6 and then flows toward the firstdetection sensor 5.

The diaphragm 6 b of the second detection sensor 6 is heated to aboutthe same temperature as that of the detection gas G through the barriercoat 6 c. That is, the temperature of the detection gas G is detected bythe thermocouple 6 e and input to the second temperature detector 3 b.

It is noted that the diaphragm 6 b and 6 c are very thin and small inarea with a diameter of some 10 to 20 mm. Therefore, the responsivenessto gas temperature detection by the thermocouple 6 e is very high, whichwill be described later.

Another feature is that the gas-contact surface of the diaphragm 6 b ofthe second detection sensor 6 is covered with the barrier coat 6 c.Therefore, even if the detection gas G contains flammable gases like H2,there will arise no reaction heating by the so-called contact catalyticactivity. As a result, the second detection sensor 6 always indicatesthe temperature of the detection gas G.

Furthermore, the barrier coat 6 c effectively prevents what is calledthe metal dusting into the detection gas G from the diaphragm 6 b andalso perfectly prevents the catalytic action of the stainless steelforming the diaphragm 6 b.

Meanwhile, the detection gas G that has passed through the seconddetection sensor 6 flows on toward the first detection sensor 5 andcomes in contact with the gas-contact surface of diaphragm 5 b.

The gas-contact surface of the diaphragm 5 b of the first detectionsensor 5 is provided with the platinum coat 5 d as mentioned above. Ifthe detection gas G contains flammable gases like H₂, the catalyticaction of the platinum coat 5 d will activate H2, causing a so-calledcontact catalytic reaction and heating the diaphragm 5 b.

The diaphragm 5 b is very thin—some 0.2 mm thick, and therefore, theheat from the contact catalytic reaction is immediately detected by thethermocouple 5 e and input into the first temperature detector 3 a.

The difference between the detected value of the first temperaturedetector 3 a and that of the second temperature detector 3 b is detectedby the temperature difference detector 3 e. From the differencetherebetween, the concentration of flammable gas in the detection gas isfound and displayed on the temperature difference display (flammable gasconcentration display) 3 f.

In the embodiment shown in FIG. 1, it is so configured that the outputsof the thermocouples 5 e, 6 e of the first detection sensor 5 and thesecond detection sensor 6 are input into the first temperature detector3 a and second temperature detector 3 b of the detector unit 3respectively. There, the temperature difference detector 3 e detects thetemperature difference between the two detectors 3 a, 3 b and convertsthe temperature difference into the concentration of flammable gas.However, the detector unit 3 may be of any construction. For example,the thermocouple output of the first detection sensor 5 and the outputof the second detection sensor 6 may be connected in reverse polarity sothat the output difference between the two first detection sensors 5, 6may be read out on the potentiometer 10 and the reading of thepotentiometer 10 may be directly converted into the concentration offlammable gas in the detection gas G.

EXAMPLE 1

FIG. 6 and FIG. 7 show the relationship between the H₂ concentrations inO₂-contained gas and the temperatures detected by the first detectionsensor 5 and the second detection sensor 6 in an actual measurementexperiment.

EXAMPLE 2

FIG. 8 shows the relationship between the O₂ concentrations inH₂-contained gas and the temperatures detected by the first detectionsensor 5 and the second detection sensor 6 in an actual measurementexperiment.

In the first example and the second example, the first detection sensor5 used in the experiments is made of stainless steel under JISdesignation SUS316L, with the diaphragm 5 b 0.2 mm in thickness, 20 mmin diameter, with the barrier coat 5 c of TiN film 2.0 μm in thickness,with the platinum coat 5 d 0.2 μm in thickness, with the thermocouple 5e of the alumel-chromel type, with the chromel tip and alumel tip heldapart 0.2 mm from each other and fixed on the diaphragm 5 b.

The second detection sensor 6 is the same as the first detection sensor5 except that only the platinum coat 5 d is absent.

As is evident from FIGS. 6 to 8, the temperature detected by the firstdetection sensor 5 is directly proportional with the concentration of H₂or O₂. It is shown that from the difference (temperature difference)between the outputs from the detection sensors 5, 6, it is possible todetect the concentration of H₂ or O₂ in the detection gas G.

EXAMPLE 3

FIG. 9 shows the results of a response characteristic or responsivenesstest of the flammable gas detector 1. The graph is actual measurementsshowing changes in temperature detection outputs of the two sensors 5, 6when the H₂ concentration in the detection gas G was suddenly raisedfrom 1.0% to 4.0%. The detection gas G was N₂ (800 SCCM)+O₂ (200 SCCM)(gas temperature 140° C.) and measurements were taken using the samefirst and second detection sensors 5, 6 as in the first and secondexamples.

As shown in FIG. 9, the temperature output (about 190.4° C.) of thefirst detection sensor 5, with the H₂ concentration of 1%, goes up to304.25° C. in some 2.3 seconds.

When the concentration of H₂ was suddenly raised from 1% to 3.0%, 2.0%and 1.5% under the same conditions, it was 2.4 seconds, 2.6 seconds and3.0 seconds respectively before the temperature detection outputs of thefirst detection sensor 5 reached 90% of the final temperature.

As is clear from FIG. 9, when the concentration of H₂ suddenly rises,the sudden increase in the gas concentration can be detected 2 to 3seconds after that (i.e. with the time lag of 2 to 3 seconds). Thus, itis confirmed that the flammable gas detection sensor 2 is high inresponse characteristic.

In the experiments shown in FIGS. 6 to 8, detection gases G containingH₂ and O₂ were used, and measurements of the concentrations of H₂ and O₂were described. The detection sensor 2 according to the presentinvention so works that the flammable gas is activated and subjected tocombustion when coming in contact with the platinum coat, therebydetecting any flammable gas, even CO. Naturally, the flammable gas isnot limited to hydrogen gas.

Also, as evident from FIG. 8, needless to say, gas detection sensoraccording to the present invention can be used as sensor for detectionof the concentration of oxygen gas in flammable gases.

(Effects of the invention)

A gas detection sensor according to the invention comprises a firstdetection sensor that includes a diaphragm having a platinum coat whichcomes in contact with the flowing detection gas and a thermocouplehaving the one ends of two different kinds of metals put close to eachother and fixed on the reverse side of the diaphragm not coming incontact with the gas and a second detection sensor of the sameconstruction but without a platinum coat. Therefore, the presentinvention provides a flammable gas detection sensor very simple inconstruction in which there exists a quite linear relation between thedetection output and the flammable gas concentration. The flammable gasdetector according to the present invention is also very useful indetecting such gases as hydrogen gas remaining in the moisture taken outfrom the reactor for generation of moisture for semiconductormanufacturing facilities.

Also, as sensor according to the invention is capable of detecting, withhigh precision, the concentration of oxygen gas in the detection gas Gcontaining flammable gas and is useful in detecting such gases as oxygengas remaining in the moisture taken out from the reactor for generationof moisture.

It is also noted that the diaphragms of the respective detection sensorsare made of stainless steel with the diaphragm gas-contact surfacecoated with a barrier coat as of TiN. And it has been confirmed that theadhesion between stainless steel and the barrier coat is extremelystrong and will undergo little change with time. In the gas detectionsensor according to the present invention, the platinum coat maintainsits stable catalytic activity for a long time with no fear of theplatinum coat and the barrier coat peeling off. Thus, the gas detectionsensor permits measurement with high precision of the concentration ofgas for a long time with no possibility of contaminating the high-puritygas.

Furthermore, when two detection sensors are mounted within the sensorblock, it is the barrier coat with a very strong adhesion provided onthe diaphragm and a coating film layer made up of the barrier coat and aplatinum coat that are exposed in the gas flow passage.

Thus, with no fear at all of the diaphragm 5 b getting damaged by theflowing detection gas, the gas detection sensor permits reliable andhigh precision measurement of the concentration of gas for a long time.Besides, the gas detection sensor according to the present invention isvery simple in construction, which substantially reduces the dead spacewithin the detection sensor. That improves gas exchangeability.

As set forth above, the present invention is excellent inpracticability.

What is claimed is:
 1. A flammable gas detection sensor wherein heatingof the sensor by contact catalytic reaction of a flowing flammabledetection gas gives off a detection signal of the flammable gas, saidflammable gas detection sensor comprising: a first detection sensorincluding a diaphragm having a platinum coat on a side thereof withwhich the flowing flammable detection gas comes in contact and athermocouple having respective ends of two different kinds of metalsplaced close to each other and fixed on a reverse side of the diaphragmnot coming in contact with the flowing flammable detection gas and whichis heated by contact catalytic reaction of the flowing flammabledetection gas, and a second detection sensor including a diaphragmhaving a surface coming in contact with the flowing flammable detectiongas and a thermocouple having respective ends of two different kinds ofmetals placed close to each other and fixed on a reverse side of thediaphragm not coming in contact with the flowing flammable detection gasand which detects the temperature of the flowing flammable detectiongas.
 2. A gas detection sensor for detection of oxygen gas present inflammable gases wherein the heating of the sensor by contact catalyticreaction of a flowing flammable detection gas gives off a detectionsignal of oxygen gas in the detection gas, said gas detection sensorcomprising: a first detection sensor including a diaphragm having aplatinum coat on a side thereof with which the flowing flammabledetection gas comes in contact and a thermocouple having respective endsof two different kinds of metals placed close to each other and fixed ona reverse side of the diaphragm not coming in contact with the flowingflammable detection gas and which is heated by the contact catalyticreaction of the flowing flammable detection gas, and a second detectionsensor including a diaphragm having a surface coming in contact with theflowing flammable detection gas and a thermocouple having respectiveends of two different kinds of metals placed close to each other andfixed on a reverse side of the diaphragm not coming in contact with theflowing flammable detection gas and which detects the temperature of theflowing flammable detection gas.
 3. A gas detection sensor as defined inclaim 1 wherein the diaphragms of the first detection sensor and thesecond detection sensor are made of stainless steel and have a barriercoat coated on the surfaces of said respective diaphragms contacted bysaid flowing flammable detection gas.
 4. A gas detection sensor asdefined in claim 3 wherein the barrier coat is a barrier coat made ofoxide or nitride.
 5. A gas detection sensor as defined in claim 1wherein the respective thermocouples are thermocouples made of chromeland alumel.
 6. A gas detection sensor as defined in claim 1 wherein astainless steel sensor block is provided with an inlet and an outlet forthe flowing flammable detection gas, a gas flow passage through whichthe inlet and outlet communicate with each other, and first and seconddetection sensor insertion ports communicating with the gas flowpassage, wherein the first detection sensor and the second detectionsensor are fit into the first and second detection sensor insertionports, respectively, provided in the stainless steel sensor block, withthe surfaces of their respective diaphragms contacted by said flowingflammable detection gas faced with the gas flowing passage, in such away that the respective detection sensor insertion ports are sealedairtight with said inserted diaphragms.
 7. A gas detection sensor asdefined in claim 2 wherein the diaphragms of the first detection sensorand the second detection sensor are made of stainless steel and have abarrier coat coated on the surfaces of said respective diaphragmscontacted by said flowing flammable detection gas.
 8. A gas detectionsensor as defined in claim 7 wherein the barrier coat is a barrier coatmade of oxide or nitride.
 9. A gas detection sensor as defined in claim2 wherein the respective thermocouples are thermocouples made of chromeland alumel.
 10. A gas detection sensor as defined in claim 2 wherein astainless steel sensor block is provided with an inlet and an outlet forthe flowing flammable detection gas, a gas flow passage through whichthe inlet and outlet communicate with each other, and first and seconddetection sensor insertion ports communicating with the gas flowpassage, wherein the first detection sensor and the second detectionsensor are fit into the first and second detection sensor insertionports, respectively, provided in the stainless steel sensor block, withthe surfaces of their respective diaphragms contacted by said flowingflammable detection gas faced with the gas flowing passage, in such away that the respective detection sensor insertion ports are sealedairtight with said inserted diaphragms.